A strain gage of a sheet-type has been conventionally known, in which a strain gage element pattern is formed on a flexible gage base (base sheet). Normally, in this type of strain gage, a tab pattern is formed on the base sheet. The tab pattern has a pair of tab main portions and a pair of connecting portions formed continuously from one edge of the respective tab main portions. The strain gage has a strain gage element pattern, which is formed by a thin conductive strip in a zig-zag pattern of parallel lines, being formed to be connected between the connecting portions. A pair of lead wires for electrically connecting the strain gage to an external device is connected to the tab main portions by soldering.
Since the strain gage is attached to a region of a member, where a strain is caused by an external force, for detecting (measuring) the strain, a mechanical strain (stress) is repeatedly applied to the strain gage itself. Accordingly, a metal fatigue may occur in a short period of time, causing the tab main portion located at an end of the soldered lead wire to have a crack or a disconnection.
There is known a strain gage having a configuration capable of coping with these problems. JP-A-5-026607 discloses an example of such strain gage provided with: a gage base bonded onto an object to be measured; a strain-sensitive resistor; and gage leads, and an elastic member is interposed between the strain gage and a terminal, and the gage leads are attached onto the elastic member in a bent state.
Another example of strain gage, disclosed in JP-B-UM-3040684, is configured to have: a narrow gage element pattern portion which is made of a resistive material and detects a strain to indicate a resistance change responsive to this strain; at least one pair of wide gage tab pattern portions to which gage leads are connected; and at least one pair of connection pattern portions for electrically connecting these gage tab pattern portions to respective ends of the gage element pattern portion, in which the gage element pattern portion, the pair of gage tab pattern portions, and the pair of connection pattern portions are bonded onto a gage base made of a flexible insulating material, wherein slits for distributing stress concentration to prevent disconnections are formed at regions of the gage tab pattern portions, which are connected to the connection pattern portions.
However, the above-described conventional strain gages and a method for connecting tab patterns and lead wires have the following problems.
Firstly, a path from the gage element pattern portion to the lead wires tends to be long because the gage lead, which is bent to have some margins, is added in the configuration disclosed in the publication JP-A-5-026607, and a U-shaped path is added in the configuration disclosed in the publication JP-B-UM-3040684. Accordingly, the addition of such paths causes not only an increase in complexity of geometric structure of a pattern and in size of the entire strain gage, but also an increase in the overall cost due to designing the strain gage with a new pattern.
Secondly, the above-described conventional strain gages are disadvantageous in terms of electrical performance because of those additional paths. To be more specific, since the conductive path other than the gage element pattern portion becomes long, an electrical resistance is deteriorated, which contributes to a reduction in the efficiency of signal transmission associated with electric loss increase and degradation in S/N characteristics, and which is also likely to become a cause of error with respect to a normal signal obtained from the gage element pattern portion.